1. Field of the Invention
The field of the invention relates to concrete, cellular concrete, and concrete aggregate materials containing slag. Methods and compositions relating to the same are provided herein.
2. Background of the Art
Concrete is a material used primarily for construction. Concrete generally consists of cement, aggregate, water, and admixtures. Through the process of hydration, concrete hardens after it has been placed. Concrete is generally mixed with water and placed as either freshly mixed concrete or as a precast element at the construction site. Admixtures include, for example, colorants, plasticizers, corrosion inhibitors, hardening accelerators, hardening retarders, air-entrainers, and bonding agents. Aggregate may be, for example, but is not limited to, a combination of sand, gravel, crushed stone, fly ash, granulated ground blast furnace slag, or recycled concrete.
Concrete is a construction material that consists of cement (commonly Portland cement), aggregate (generally gravel or stone, and sand), water and admixtures, and is commonly known as the most used man-made material in the world. Precast concrete is concrete that is cast in a reusable mold, then moved to the location of use. Precast concrete may be used, for example, in soundproofing, in constructing a building facade, or as a structural, loadbearing element. Aggregates used in concrete are typically obtained from alluvial deposits (gravel and sand) or mined (limestone, granite and other types of stone). Depending upon the aggregate size(s) required the gravel or stone is either screened or crushed to the desired size. It is not unusual for concrete that is removed from pavement and other normal concrete uses to be recycled by crushing and sizing so that it can be reused as a coarse aggregate in regular concrete.
Cellular concrete is a material produced throughout the world, typically including cement, sand, water, various admixtures, fiber, and air. For example, the properties of cellular concrete are reported in the American Concrete Institute's Manual of Concrete Practice, ACI 523.1R-92 and 523.2R-96, incorporated by reference herein. A number of companies manufacture and sell foam generators and foaming agents used in the production of cellular concrete.
Cellular lightweight concrete (“CLC”) was reported in two 2004 issues of Precast Concrete News Magazine in a two-part article by William Abbate. CLC is commonly produced in North America for a variety of applications, including for use as a structural building material. It has been used in limited amounts in the precast industry.
Standards, guides, specifications and other information about cellular concrete are available from ACI, ASTM, and similar authoritative sources around the world. Cellular concrete is produced in two basic forms: autoclaved and conventionally cured. The autoclaved version of cellular concrete is commonly known throughout the world as AAC, ACC, or ALC. These abbreviations refer to autoclaved aerated, cellular, and lightweight concrete respectively, and are essentially the same product. By autoclaving, strength gain is greatly accelerated allowing full strength to be achieved within a number of hours.
The manufacturing process is very different between CLC and autoclaved aerated concrete. Whereas autoclaved cellular concrete is cast into molds and cut into its various shapes before it hardens, CLC can be placed into forms just like regular concrete, offering greater versatility and a much simpler manufacturing process. Additionally, whereas typical autoclaved cellular concrete plants contain specialized equipment and can cost as much as $35 million or more, cellular concrete can be produced in existing precast plants with a very modest initial investment.
Cellular concrete is used throughout North America in lightweight insulating roof decks, floor screeds, and geotechnical engineered fills. These applications of CLC are usually at unit weights below 50 pounds per cubic foot (pcf). While some building construction currently utilizes CLC, market penetration is small thus far. Throughout the world CLC has been used as structural wall systems in literally hundreds of thousands of homes, commercial, and governmental buildings. In many of these structures, the CLC is above 50 pcf, usually in the 70 to 85 pcf range.
As with regular weight concrete, CLC can be cast into virtually any shape. CLC, like any other type of concrete, is better suited for some applications than others. CLC may have one or more of the following attributes when compared to other types of concrete:                Greatly reduced weight        Increased fire resistance        High thermal insulation values        Excellent sound resistance        Woodworking properties (with most mix designs)        Enhanced environmentally friendliness        Permits single component wall construction        Saves material and labor costs        
Cellular concrete generally consists of cement, sand, water, and preformed foam, with a water-cement ratio of around 0.50. Most precast mixes will also contain synthetic fibers. Compatible superplasticizer and accelerating admixtures are available from some manufacturers. One caution when using any admixtures in CLC is to ensure they are compatible with the foaming agent so that the integrity of the cellular structure is not compromised. Depending on requirements, lightweight and other aggregates can be used as part of the mix design to achieve greater strength at lower weights if needed. Note that when using larger aggregates, the ability to saw, nail, and screw into the material can be affected.
Various types of equipment including pan, ribbon, and drum mixers are routinely used in the production of CLC. Once the mix design for a product is determined, production of cellular concrete is a relatively simple matter.
Two things must be monitored to produce consistent product. First, it is important to fully satisfy the cement's water demand before adding preformed foam to ensure the foam maintains its integrity. A water-cement ratio (w/c) of about 0.40 is usually recommended to achieve this. As mentioned previously, typical mix designs aim for a final w/c of around 0.50.
Second, there are a number of simple quality checks that can be performed to monitor density, a key measurement to ensure consistent quality during production. A quick and easy check is to take a fresh unit weight.
Most CLC mixes are rather fluid and easy to place into forms. If the mix contains no coarse aggregate, which is usually the case, internal vibration has little effect. Should it be required, external vibration of the forms is recommended. CLC can be conveyed into forms in the same manner as any high-slump concrete.